1. Technical Field
The invention relates generally to methods for making carbon nanotubes, and more particularly to a method for making an array of carbon nanotubes.
2. Discussion of Related Art
Carbon nanotubes were discovered by S. Iijima in 1991, they are very small tube-shaped structures, each essentially having composition similar to that of a graphite sheet rolled into a tube. Theoretical studies show that carbon nanotubes exhibit either metallic or semiconductive behavior depending on the radii and helicity of the tubules. Carbon nanotubes have interesting and potentially useful electrical and mechanical properties, and have many potential uses in electronic devices. Carbon nanotubes also feature extremely high electrical conductivity, very small diameters (much less than 100 nanometers), large aspect ratios (i.e. length/diameter ratios) (greater than 1000), and a tip-surface area near the theoretical limit (the smaller the tip-surface area, the more concentrated the electric field, and the greater the field enhancement factor). These features make carbon nanotubes ideal candidates for electron field emitters, white light sources, lithium secondary batteries, hydrogen storage cells, transistors, and cathode ray tubes (CRTs).
Generally, there are three methods for manufacturing carbon nanotubes. The first method is the arc discharge method, which was first discovered and reported in an article by Sumio Iijima entitled “Helical Microtubules of Graphitic Carbon” (Nature, Vol. 354, Nov. 7, 1991, pp. 56-58). The second method is the laser ablation method, which was reported in an article by T. W. Ebbesen et al. entitled “Large-scale Synthesis of Carbon Nanotubes” (Nature, Vol. 358, 1992, pp. 220). The third method is the chemical vapor deposition (CVD) method, which was reported in an article by W. Z. Li entitled “Large-scale Synthesis of Aligned Carbon Nanotubes” (Science, Vol. 274, 1996, pp. 1701).
In the arc discharge method, a carbon vapour is created by an arc discharge between two carbon electrodes with or without catalyst. Carbon nanotubes self-assemble from the resulting carbon vapour. In the laser ablation technique, high-powered laser pulses impinge on a volume of carbon-containing feedstock gas (methane or carbon monoxide). Carbon nanotubes are thus condensed by the laser ablation and are deposited on an outside collector. However, the carbon nanotubes produced by the arc discharge and the laser ablation vary greatly in diameter and length, with little control over the dimensions of the resulting product. Moreover, poor carbon nanotube yield and prohibitive cost involved in making the device mean that the two methods difficult to scale up to suit industrial production.
In the chemical vapour deposition (CVD) method, carbon filaments and fibers are produced by thermal decomposition of a hydrocarbon gas on a transition metal catalyst in a chemical vapour deposition reaction chamber. In general, the chemical vapour deposition process results in both multi-walled nanotubes (MWNTs) and single-walled nanotubes (SWNTs) being produced. Compared with the arc discharge method and laser ablation method, the chemical vapour deposition method is a more a simple process and can easily be scaled up for industrial production. However, the carbon nanotubes manufactured by the chemical vapour deposition process aren't bundled to form an array, thus the CVD process can't assure both quantity and quality of production.
In view of the above, another method, such as a thermal chemical vapor deposition method is disclosed where an array of carbon nanotubes are formed vertically aligned on a large-size substrate. The thermal CVD method includes the steps of forming a metal catalyst layer on a substrate; etching the metal catalyst layer to form isolated nano-sized catalytic metal particles; growing carbon nanotubes from said isolated nano-sized catalytic metal particles by the thermal chemical vapor deposition (CVD) process; and purifying the carbon nanotubes in-situ.
The carbon nanotubes formed by the above-described method are vertically aligned on the substrate. However, the above-described method is a rather complicated process. Furthermore, excess amorphous carbon lumps and metal catalyst lumps are also produced along with the carbon nanotubes and adhered to inner or outer sidewalls of the carbon nanotubes. Thus, the above-described method needs complicated purification processes. Moreover, the above-described method is performed at a temperature in the range from 700° C. to 1000° C., thus it is difficult to mass synthesize the carbon nanotubes due to the high synthesis temperature. Furthermore, the carbon nanotubes formed in the above-described method are generally comprised of MWNTs and SWNTs. These mixed carbon nanotubes do not sufficiently exhibit the useful properties of a single-type array of carbon nanotubes.
What is needed, therefore, is a method for making a carbon nanotube array that is easy to operate, uses a simple process, and can form high purity carbon nanotube arrays.